The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. Before the engines are to be turned, the APU is started. Once the APU is running, it provides the power to start the aircraft's main engines.
APUs are also used to run accessories while the engines are shut down. This allows the cabin to be comfortable while the passengers are boarding before the aircraft's engines are started. Electrical power is used to run systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing crews to operate hydraulic equipment prior to main engine start up. This function can also be used, on some aircraft, as a backup in flight in case of the loss of main engine power or hydraulic pressure.
APUs fitted to extended-range twin-engine operations (ETOPS) aircraft are an important backup system, as they supply backup electricity, compressed air and hydraulic power in the highly unlikely, yet postulated event of a loss of main engine power or a failed main engine generator. As such, operators flying ETOPS legs are required to track and record oil consumption rates to ensure the APU is always serviced with sufficient oil for the duration of the flight mission. To support that need, APU controllers will display oil quantity on a flight deck display.
Certain control systems have the capability of displaying quantity in discrete units, e.g., quarts or liters, or volume; however, displaying a consistent and accurate oil quantity while the APU is running is problematic. This is because these systems inadequately account for APU startup or shutdown gulp. To acquire a consistent oil quantity measurement with sufficient accuracy to permit ETOPS operation requires the aircraft maintenance operators to shut down the APU. Unfortunately, shutting down the APU is not preferred due to the increased time, work and inconvenience to restart the APU, e.g., hook up ground power, get a ground cart, etc. Therefore, aircraft manufacturers and operators are consistently looking for improved APU oil quantity measurement systems to support maintenance activities and reduce aircraft downtime.
Accordingly, it is desirable to provide methods and systems to determine and indicate an accurate oil quantity of an APU, particularly as the APU is running. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.